The European Telecommunications Standard Institute—Digital Mobile Radio (ETSI-DMR) is a direct digital replacement for analog Private Mobile Radio (PMR). DMR is a scalable system that can be used in unlicensed mode (in a 446.1 to 446.2 MHz band), and in licensed mode, subject to national frequency planning. Any of the ETSI standards or specifications referred to herein may be obtained by contacting ETSI at ETSI Secretariat, 650, route des Lucioles, 06921 Sophia-Antipolis Cedex, FRANCE.
DMR promises improved range, higher data rates, more efficient use of spectrum, and improved battery. Features supported include fast call set-up, calls to groups and individuals, short data and packet data calls. Supported communications modes include individual calls and group calls provided via a direct communication mode among the radios operating within the network. Other important DMR functions such as emergency calls, priority calls, short data messages and Internet Protocol (IP)-packet data transmissions are also supported.
Direct mode, more generally, is a mode of operation where radios may communicate within a network without the assistance of one or more infrastructure equipment (e.g., base stations or repeaters). A radio, as used herein, can be mobile and/or fixed end equipment that is used to obtain direct mode communications services. Direct mode can provide a more efficient and less costly communication system operation than repeater mode operation.
The ETSI-DMR standard provides for 12.5 Kilohertz (KHz) operation in direct mode. The 12.5 KHz operation refers to 12.5 KHz spectral efficiency in which there are two communication paths per 12.5 KHz of radio frequency (RF) spectrum. The 12.5 direct mode utilizes 27.5 millisecond (msec) pulsed (every 60 msec) radio transmissions on each of the logical channels. In the 12.5 direct mode of operation, radios transmit asynchronously and radios within range of the transmission synchronize themselves to that transmission for the purposes of receiving the transmission, but any transmissions in response to the first transmission are transmitted asynchronously.
Other direct mode protocols, perhaps consistent with the Project 25 (P25) standard defined by the Association of Public Safety Communications Officials International (APCO) and standardized under the Telecommunications Industry Association (TIA), or with the terrestrial trunked radio (TETRA) standard also defined by the ETSI, may operate in a similar manner and may be used in addition to or in place of the DMR protocol. Communications in accordance with any one or more of these direct mode communication standards, or other standards, may take place over physical channels in accordance with a TDMA (time division multiple access) protocol, perhaps in combination with an FDMA (frequency divisional multiple access) or CDMA (code division multiple access) protocol.
In half duplex TDMA direct mode communications systems, voice and/or data moves in only one direction at a time (source to target(s)), as compared to full duplex, in which voice and/or data can move in both directions (e.g., source to target(s) and target(s) to source). For example, the ETSI-DMR 6.25e standard implements an N:1 slotting ratio, where N=2, such that two half duplex TDMA direct mode calls can occur substantially simultaneously on a single respective direct mode frequency.
Prior efforts to implement full duplex calls in a TDMA direct mode communications system conventionally provided a second separate frequency on which to provide a return audio and/or data path from the target to the source that is time-aligned with the forward path. However, given the short guard intervals (˜2.5 ms) between time slots in accordance with the ETSI-DMR 6.25e standard, a typical radio cannot switch between a first direct mode frequency to transmit on a first time slot (TS1) and a second direct mode frequency to receive on a second time slot (TS2) within the guard time interval provided under the standard. While the incorporation of a second synthesizer in the radio could alleviate some of the difficulty, the addition of a second independent synthesizer substantially increases costs to manufacture a radio, and is thus a less than optional solution.
Accordingly, what is needed is an improved method, system, and device for providing full duplex voice and/or data communications services in a direct mode N:1 TDMA communications systems that does not require each radio to switch its synthesizer between a transmit frequency and a different receive frequency, and vice versa, within an amount of time between immediately adjacent slots in the direct mode N:1 TDMA protocol.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.